The present invention relates generally to operations performed and equipment utilized in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides a deep set safety valve.
It is sometimes desirable to set a safety valve relatively deep in a well. For example, a safety valve may be set at a depth of approximately 10,000 ft. However, operating a safety valve at such depths presents a variety of problems which tend to require expensive measures to overcome.
For example, a typical safety valve is operated by displacing a piston of the safety valve in response to a differential between pressure in a control line connected to the safety valve and pressure in a tubing string in which the safety valve is interconnected. In such situations, the control line is generally designed to withstand a pressure in excess of the greatest pressure predicted in the tubing string, plus a pressure needed to compress a biasing device, such as a spring. This usually requires a relatively high pressure rating for the control line, which significantly increases the cost of the safety valve installation, particularly in subsea environments where the control line may extend for many thousands of feet along the seabed.
Another problem associated with use of this type of deep set safety valve is the presence of a dynamic seal between portions of the valve at control line pressure and portions of the valve at tubing string internal pressure. A leak past the dynamic seal could possibly permit well fluids (such as liquid hydrocarbons or gas) in the tubing string to enter the control line.
One proposed solution is to use a second control line to balance the pressure in the other control line. In this type of safety valve, the piston displaces in response to a differential between pressures in the two control lines. This significantly eliminates the consideration of tubing string internal pressure in calculating the required pressure rating of the control line for normal operation of the valve. However, this method requires the installation of two control lines, which is very costly.
This type of safety valve also typically has one or more dynamic seals isolating the tubing string internal pressure from portions of the safety valve at control line pressure. Thus, the control lines are generally required to withstand the tubing string internal pressure for safety reasons, in case a leak past one of the dynamic seals occurs.
Another proposed solution is to use a safety valve which includes a gas chamber charged to a predetermined pressure. The piston of the safety valve displaces in response to a differential between control line pressure and the gas chamber pressure. This method also substantially eliminates the consideration of tubing string internal pressure in the control line pressure rating for normal operation of the valve, but this type of safety valve also uses dynamic seals to separate portions of the valve at tubing string internal pressure from portions at control line pressure and/or gas chamber pressure.